The present invention relates in general to the timing system for internal combustion engines, hereinafter referred to as i.c. engines, especially for motor vehicles, comprising at least one distribution shaft provided with cam means and at least one valve operated by the said cam means.
Recent years have seen motor car research concentrate ever greater interest on obtaining torque curves such as to keep on improving the comfort and elasticity in motion of motor vehicles equipped with this type of i.c. engine. Nevertheless, motor vehicles that are ever more spacious and ever more equipped with safety systems, be they active or passive, and therefore of ever greater weight, together with the need for making more economic use of energy, also call for the highest possible values of the driving torque to be obtained from engines of an ever smaller cubic capacity.
With a view to obtaining all this, the makers of i.c. engines are at present working along two development lines, sometimes at one and the same time.
The first of these envisages a twofold possibility of the distribution diagramme: normally the suction phase is advanced or retarded with respect to the exhaust phase, or a second suction valve is brought into operation at a certain speed of rotation, this with a view to obtaining at least two different distribution patterns in the course of engine use (one more suitable at low rotation regimes, the other for obtaining the maximum power). With this solution, however, one obtains no more than a compromise, because the total valve opening angle, which necessarily remains constant, imposes limits on both the first and the second case.
At low regimes and/or low loads the valve opening is retarded (but, consequently, this also applies as regards its closure) so as to avoid the discharge of unburnt gases in the exhaust phase. But when this is done, the pumping work has to be increased and there is a backflow of the gases already sucked in on account of the low intake speed. As a result one obtains a low volumetric efficiency and a smaller mechanical efficiency.
At higher regimes and loads the opening of the suction valve is advanced to facilitate the filling of the cylinder, but the fact that the closure of this valve is likewise advanced almost wholly excludes the possibility of utilizing the inertia of the fluid column in the intake duct, which would have the effect of producing a higher degree of filling that costs nothing in terms of mechanical losses.
Furthermore, the instantaneous phase variation produces a torque curve that is humped rather than homogeneous, because it is the resultant of two different curves that have apices well apart from each other.
The second line is the one that aims at complicated intake manifolds, designed in such a way that, inserting two or three butterfly valves at particular points and combining the opening of one with the opening and/or closure of the others, there are created different lengths of the intake ducts, each suitable for obtaining the best possible torque, though necessarily only for a limited range of engine rotation regimes, thus exploiting both the inertia of the gases and the pressure waves set up by the opening and closing of the valves. The programmed intervention of these combinations gives risexe2x80x94always within a particular range of engine rotation regimesxe2x80x94to a more favourable driving torque, but in this case, once again, the resulting curve is not homogeneous and is characterized by two or three humps corresponding to the various maximum values.
At least at low loads and engine rotation regimes, moreover, it has long since become indispensablexe2x80x94and therefore the subject of researchxe2x80x94to have a xe2x80x9cpoorxe2x80x9d air-petrol mixture, that is to say, a fuel well below the stoichiometric optimum, and thus to diminish both the specific fuel consumption and atmospheric pollution.
The limit to the degree of poverty is however conditioned by the possibility of igniting the mixture (ignition must remain certain if damage to the catalytic silencer/converter is to be avoided), and ignition becomes more efficient as speed and turbulence become greater in both the intake ducts and the combustion chamber. But at low regimes and/or low loads greater speeds can be obtained only by reducing the passage section.
With a view to obviating the aforesaid disadvantages, the present invention has the purpose of attaining one or more of the objectives listed below:
Provide a variator for the feed system that will enable the i.c. engine to operatexe2x80x94at all rotation regimes and at all load conditionsxe2x80x94with the distribution diagramme and the gas passage section most favourable for obtaining the maximum torque, the minimum specific consumption and the lowest degree of atmospheric pollution.
Make it possible to vary not only the angles of both the beginning of the opening and the end of the closing of the valves, but also their lifts, in a gradual and continuous manner from zero to the maximum permitted by the cams that produce them.
When there are several suction valves, make it possible for these valves to operate with different opening times and thus to obtain even at the xe2x80x9cminimumxe2x80x9d and at very small loads gas speeds sufficiently high to provide adequate turbulence for ensuring ignition and regular functioning and also to satisfy the ever present requirements of low specific consumption and degree of pollution.
Eliminate the butterfly choking shutter (choke), cause of not by any means negligible pumping losses and therefore overall engine efficiency, especially at low loads, which represent also the most commonly used engine condition.
Always with a view to reducing the engine""s specific consumption, make it possible for the engine to be managed in such manner as to get it to work also with a number of cylinders smaller than their total number when only a limited power is required.
Obtain from an engine of relatively limited cylinder capacity the same performance associated with one of greater capacity, with corresponding benefits in terms of size, weight and consumption, obtain an appreciable overall efficiency improvement.
According to the invention, the aforesaid purposes can be obtained by means of the possibility of mechanically varyingxe2x80x94either directly or via an electronic control, but in either case connected with the gas control of the i.c. enginexe2x80x94the position of appropriate oil passage holes in appropriately designed hydraulic tappet (valve follower) systems and thus to determine valve lifts and distribution diagrammes that can be gradually and continuously varied from zero to the maximum permitted by the cam and thus to introduce into the cylinder, and also at the most favourable moment, the quantity of air or of air-fuel mixture that is optimal no matter what the required running condition, and this without having to make use of a so-called xe2x80x9cbutterflyxe2x80x9d choke valve.
In greater detail, the invention envisages a timing system of i.c. engines, especially for motor vehicles, comprising at least one distribution shaft provided with cam means, at least one suction or exhaust valve operated by the said cam means, and control means for providing variable lifts and timings of the said valve, in which between the said cam means and the valve there is arranged a hydraulic tappet system including a cup member and a piston delimiting therebetween a chamber that communicates with inlet passage means for feeding oil from a hydraulic circuit, wherein closure of said inlet passage means is performed by the said cup member as a result of displacement thereof towards the said piston operated by the said cam means, characterized by the fact that the position of the said inlet passage means can be selectively regulated in such a way as to correspondingly vary the moment at which they become closed by the said cup member and, consequently, retard or, respectively, advance the opening of the said valve i.e. decrease or increase the lift thereof, respectively.